This invention relates to copper-containing materials. Copper containing compositions, wherein some or all of the copper is in the form of elemental copper, or in the oxide form, i.e. as cupric and or cuprous oxides, or in the form of other copper species, e.g. sulphides, basic carbonates and the like, are widely used in industrial processes as catalysts or sorbents. For example compositions wherein some or all of the copper is in the elemental form are often used as catalysts for reactions involving hydrogen. As examples there may be mentioned the shift reaction wherein carbon monoxide is reacted with steam to form carbon dioxide and hydrogen; alcohol synthesis reactions wherein a mixture of hydrogen and carbon monoxide and/or carbon dioxide is reacted to form methanol or higher alcohols; hydrogenation reactions; and hydrogenolysis of esters. Compositions wherein some or all of the copper is in the form of elemental copper, copper oxides, copper hydroxide or basic copper carbonate, can be used as sorbents for purification of gases and liquids to remove contaminants such as sulphur compounds. Compositions wherein the copper is in the form of copper sulphides can be used as sorbents for the removal of contaminants such as arsenic compounds and mercury from gases and liquids.
For such applications it is generally desirable that the copper species is present in a highly dispersed form so that the active species is readily contacted with the reactants or material being treated. The degree of dispersion of the copper species can be assessed by determining the exposed surface area of the copper (after reduction of the copper species to elemental copper) per gram of copper. A high copper surface area per gram of copper implies a high degree of dispersion. The copper surface area is conveniently determined by the nitrous oxide decomposition method, for example as described by Evans et at in xe2x80x9cApplied Catalysisxe2x80x9d, 7, (1983), pages 75-83xe2x80x94a particularly suitable technique is described in EP 0 202 824.
It is known that compositions having, upon reduction, a high metal surface area per gram of metal may be made by impregnating a transition alumina support with a metal ammine carbonate solutions, followed by heating to decompose the ammine carbonate. Thus EP 0 092 878 describes the production of nickel on alumina compositions and WO 96 04 072 describes the production of the analogous cobalt compositions. However, the degree of dispersion of a metal obtained by impregnation of a support, such as alumina, with a solution of a salt or complex of the metal, depends on the ease of decomposition of the complex or on the solubility of the salt. If the salt is too soluble or the complex is too stable, agglomerates of the metal species are liable to be formed rather than a thin layer of the decomposition products upon the surfaces of the pores of the alumina support. Such agglomerates will give, upon reduction to the metal, materials having a relatively low metal surface area. The nickel and cobalt ammine carbonate complexes as employed in the aforesaid EP 0 092 878 or WO 96 04 072 have a relatively low stability. Thus they decompose very readily.
We have found that although copper ammine carbonate complexes are significantly different and are far more stable than the cobalt or nickel analogues, surprisingly high surface area copper materials can be obtained by this route. Whereas cobalt and nickel form hexa-ammine complexes, copper forms tetra-ammine complexes. The complex constants for the copper, cobalt and nickel ammonia complexes are as follows:
As the composition is heated to decompose the ammine complex, the cobalt and nickel materials precipitate readily when only some of the ammonia has been driven off. On the other hand, on heating the copper complexes, since they are much more stable, it would be expected that the copper would remain longer in solution during evaporation of the water and ammonia and would be liable to be deposited unevenly at the locations where the last of the water is removed, and hence give rise to aggregates of the copper composition rather than as the desired thin coating on the pores of the alumina support. Indeed, in EP 0 259 911 it was proposed to make metal/alumina catalysts by combining an ammoniacal solution of a salt such as a carbonate of the metal with an aqueous solution of an aluminium compound and heating the mixture to boiling, or near boiling, to precipitate a mixed basic carbonate of the metal and aluminium. Whereas this method gave compositions which, when reduced, had a high metal surface area per gram of metal when the metal was nickel, the metal surface area was only 1-20 m2 per gram of metal when the metal was copper.
Accordingly the present invention provides a process for the manufacture of a composition comprising a copper compound supported on a porous transition alumina comprising impregnating a porous transition alumina support with an aqueous solution of a copper ammine carbonate complex, draining off any excess of the impregnating solution, and then heating the impregnated support to a temperature above 80xc2x0 C. to decompose the complex thereby depositing a basic copper carbonate compound on the surfaces of the pores of the transition alumina support.
The transition alumina may be of the gamma-alumina group, for example a eta-alumina or chi-alumina. These materials may be formed by calcination of aluminium hydroxides at 400-750xc2x0 C. and generally have a BET surface area in the range 150-400 mu2/g. Alternatively, the transition alumina may be of the delta-alumina group which includes the high temperature forms such as delta- and theta-aluminas which may be formed by heating a gamma group alumina to a temperature above about 800xc2x0 C. The delta-group aluminas generally have a BET surface area in the range 50-150 m2/g. The transition aluminas contain less than 0.5 mol of water per mole of Al2O3, the actual amount of water depending on the temperature to which they have been heated. The support should be porous, preferably having a pore volume of at least 0.2 ml/g, particularly in the range 0.3 to 1 ml/g.
The support may be in powder form, but is preferably in the form of shaped units, for example approximate spheres, pellets, cylindrical tablets, agglomerates. The shaped units preferably have a minimum dimension of at least 1 mm, and preferably have maximum and minimum dimensions in the range 1 to 15 mm, preferably 3 to 10 mm. The maximum dimension is preferably not more than 3 times the minimum dimension. Where a powdered alumina is employed, the alumina preferably has a surface-weighted mean diameter in the range 1 to 100 xcexcm. [The term surface-weighted mean diameter D[3,2], otherwise termed the Sauter mean diameter, is defined by M. Alderliesten in the paper xe2x80x9cA Nomenclature for Mean Particle Diametersxe2x80x9d; Anal. Proc., vol 21, May 1984, pages 167-172, and is calculated from the particle size analysis which may conveniently be effected by laser diffraction for example using a Malvern Mastersizer]. Alternatively the support may be in the form of a monolith, e.g. a honeycomb. In the latter case, the honeycomb may be formed from a ceramic or metal support with a coating of the transition alumina.
The amount of the copper ammine carbonate complex employed is preferably such that the composition has a copper to aluminium atomic ratio in the range 0.025 to 0.5, corresponding to a copper content, in a binary copper species/alumina composition, (after reduction of the copper species to elemental copper) of about 3 to 40% by weight.
The shaped units of the invention may be made by impregnating the support with an aqueous solution of a copper ammine carbonate complex and, after draining off any excess of impregnation solution, then heating the impregnated support to decompose the copper ammine carbonate complex. Heating at temperatures above about 80xc2x0 C. is sufficient to decompose the copper ammine carbonate, with the evolution of ammonia and carbon dioxide, to give a basic copper carbonate. Heating at temperatures above about 200xc2x0 C., particularly above about 250xc2x0 C., will cause the basic copper carbonate to decompose to give a copper oxide species. The basic copper carbonate species, or the copper oxide species, can be converted to other copper species such as elemental copper by reduction or to copper sulphide by sulphidina with a suitable sulphur compound, e.g. hydrogen sulphide or a solution of an alkali or ammonium sulphide or polysulphide as is known in the art.
The copper ammine carbonate solution may be made by dissolving basic copper carbonate in an aqueous solution of ammonium carbonate containing additional ammonium hydroxide. The relative amounts should be such that the pH of the solution is in the range 7-12, preferably 8-11. The solution preferably contains 1-5, particularly 2-4, especially 1.5-2.5 moles of the copper complex per liter. As the concentration of copper increases, then generally the proportion of carbonate ions relative to hydroxide ions in the basic copper carbonate feed should be increased.
Where the transition alumina is in the form of shaped units, the shaped units may be given multiple impregnations with the copper ammine carbonate solution with heating between impregnations to effect decomposition of the copper ammine carbonate. By this method, a thin layer of the hydroxycarbonate is deposited on the surfaces of the pores of the transition alumina shaped units. On the other hand, where the alumina is employed in powder form, the transition alumina powder may be slurried with the appropriate amount of an aqueous solution of the copper ammine carbonate complex to give a product of the desired copper content. The alumina carrying the deposited copper compound is then filtered from the aqueous medium and heated. The resultant product may then be formed into shaped units, e.g. pelleted, if desired.
If it is desired to have the copper species in the form of the oxide, the product may be calcined at a temperature in the range 200-500xc2x0 C., particularly 250-450xc2x0 C. In some cases where the oxide is desired and transition alumina shaped units are given multiple impregnations, it may be desirable to calcine the impregnated material between impregnations. Where it is desired to have the copper in the form of elemental metal, the dried impregnated supports may be reduced directly with a suitable reductant, e.g. hydrogen, preferably diluted with an inert gas, at a temperature in the range 150-400xc2x0 C., particularly 200-300xc2x0 C. Alternatively the reduction may be effected after conversion of the deposited copper hydroxycarbonate to copper oxide by calcination.
Conversion of the copper metal, copper oxide or copper hydroxycarbonate to other copper species, e.g. sulphides, may be effected by known procedures.
Depending on the pore volume of the support, and the concentration of the impregnating solution employed, it is possible to produce compositions containing an amount of the copper species varying over a wide range. For example compositions having a copper species content in the range 3 to 40% by weight, expressed as copper, based on the combined weights of the transition alumina and the copper species, may be produced. For compositions having a copper to aluminium atomic ratio above about 0.09, corresponding to a copper content, in a binary copper species/alumina composition, (after reduction of the copper species to elemental copper) of about 10% by weight, multiple impregnations may be required. On reduction of the copper species, e.g. by hydrogen at temperatures ranging from 150-250xc2x0 C., compositions having a copper surface area above 40 m2 per gram of copper may be obtained. Preferably the copper surface area is above 50 m2, particularly above 60 m2, per gram of copper.
Certain compositions of a copper species on a support of transition alumina containing substantial amounts of the copper species and having, upon reduction of the copper species a high copper surface area per gram of copper are new.
Thus copper compositions obtained by impregnating gamma-alumina powder with copper nitrate followed by drying and calcination are described by Robinson et al in xe2x80x9cApplied Cataiysisxe2x80x9d, 44, (1988), pages 165-177. FIG. 5 of this paper indicates that the maximum copper surface area, per gram of unreduced catalyst, was about 9 m2/g and was given by a composition containing about 8% by weight copper (Cu/Al atomic ratio about 0.07). This corresponds to a copper surface area of about 112 m2 per gram of copper. However at higher copper contents, the copper surface area decreased. Thus at a copper content of about 24% by weight (Cu/Al atomic ratio about 0.25) the surface area was about 3 m2 per gram of unreduced catalyst, i.e. a copper surface area of about 12.5 m2 per gram of copper.
It is desirable to produce copper compositions having, upon reduction of the copper species, a high copper surface area per gram of copper, and at the same time having substantial copper contents.
In xe2x80x9cACS Division of Fuel Chemistryxe2x80x9d, 29, No. 5, (1984), pages 178-188, Chinchen et al list copper/alumina catalysts for methanol synthesis having copper contents of 20%, 40% and 60% having a copper surface areas of 11.7, 19.9 and 12.7 m2 per gram of unreduced catalyst respectively, corresponding to copper surface areas of about 58.5 m2, 49.8 m2 and 21.2 m2 per gram of copper. The method of production of these copper/alumina catalysts is not mentioned, but since they are compared with conventional copper/zinc oxide/alumina methanol synthesis catalysts they were presumably made by co-precipitation as is normal for methanol synthesis catalysts, and not by impregnating a preformed transition alumina support.
Copper catalysts having a copper surface area above 35 m2 per gram of copper are described in U.S. Pat. No. 5,302,569. These are prepared by coprecipitation of copper, zinc and aluminium compounds as e.g. carbonates followed by calcination. The proportion of alumina in the calcined catalysts is relatively small, ranging from 2 to 50 parts by weight per 100 parts by weight of copper oxide in the calcined composition. The examples describe the production of compositions which have, upon reduction, copper surface areas ranging from 54 to 76 m2 per gram of copper.
Copper catalysts having greater copper surface areas, above 70 m2 per gram of copper, are described in U.S. Pat. No. 4,863,894. Those catalysts were made by co-precipitation of copper compounds with zinc and, optionally, aluminium compounds as basic carbonates and then reducing the copper species to the elemental form without heating the basic carbonate composition to temperatures above 200xc2x0 C. However such a technique imposes difficulties in obtaining the catalyst in a suitable physical form. Thus pelleting of the basic carbonate composition gives rise to products of low strength upon reduction, while pelleting after reduction requires that the pelleting has to be effected in an inert atmosphere. The catalysts of this reference contain only a minor amount, if any, of alumina, but a significant amount of zinc and/or magnesium.
In the present invention it is preferred that the compositions have a substantial copper contents but also contain, after reduction, at least 60% by weight of alumina, and are preferably essentially free from compounds of zinc and magnesium, and have, upon reduction of the copper species, a high copper surface area per gram of copper. The compositions are preferably in the form of shaped units suitable for use as fixed bed catalysts or sorbents.
Accordingly the present invention further provides a composition comprising a copper species and porous transition alumina, preferably in the form of support units having a minimum dimension of at least 1 mm, said composition having a copper to aluminium atomic ratio in the range 0.14 to 0.5 and having, upon reduction of the copper species with hydrogen at 250xc2x0 C., a copper surface area of at least 60 m2, preferably at least 80 m2, per gram of copper.
The composition preferably has a BET surface area above 80 m2/g. Preferably the copper to aluminium atomic ratio is at least 0.16.
The products of the invention may be used as catalysts or adsorbents. With the copper species in the reduced form, they are of use as catalysts for reactions involving hydrogen. For such use in some cases it may be desirable to impregnate the product with a noble metal such as platinum, palladium, or rhodium. With the copper species in the hydroxycarbonate or oxide form they are of use in the purification of gases and liquids by the removal of sulphur compounds. With the copper species in the form of a sulphide, they are of use as absorbents for the removal of impurities such as mercury and arsines from gas and liquid streams.